The HIV infection cycle begins with the entry of the virus into the target cell. The human CD4 molecule is the primary receptor recognized by HIV. The binding of the HIV envelope glycoprotein (env) to the CD4 receptor results in the fusion of virus and cell membranes, which in turn facilitates virus entry into the host. The eventual expression of env on the surface of the HIV-infected host cell enables this cell to fuse with uninfected, CD4-positive cells, thereby spreading the virus.
Recent studies have shown that this HIV fusion process occurs with a wide range of human cell types that either express human CD4 endogenously or have been engineered to express human CD4. The fusion process, however, does not occur with nonhuman cell types engineered to express human CD4. Although such nonhuman cells can still bind env, membrane fusion does not follow. The disparity between human and nonhuman cell types exists apparently because membrane fusion requires the coexpression of human CD4 and an accessory factor specific to human cell types. Because they lack this accessory factor, nonhuman cell types engineered to express only human CD4 are incapable of membrane fusion, and are thus nonpermissive for HIV infection. To date there has been no report of any stable, nonhuman cell line that is permissive for HIV infection as a result of human CD4 and CXCR4 coexpression.
The importance of human CD4 and CXCR4 coexpression also impacts the establishment of a successful small animal model. The development of a small animal model is crucial to the study of HIV infection and the effectiveness of anti-HIV therapeutics. In recent years, researchers have bred transgenic animals having cells that express human CD4. See, for example, Dunn et al., Human immunodeficiency virus type 1 infection of human CD4-transgenic rabbits, J. Gen. Vir. 76:1327-1336 (1995); Snyder et al., Development and Tissue-Specific Expression of Human CD4 in Transgenic Rabbits, Mol. Reprod. & Devel. 40:419-428 (1995); Killeen et al., Regulated Expression of Human CD$ Rescues Helper T-Cell Development in Mice Lacking Expression of Endogenous CD4, EMBRO J. 12:1547-1553 (1993); Forte et al., Human CD4 Produced in Lymphoid Cells of Transgenic Mice Binds HIV p120 and Modifies the Subsets of Mouse T-Cell Populations, Immunogenetics 38:455-459 (1993). These animals, however, have low susceptibility to HIV infection, presumably because of the lack of CXCR4 expression. To date, there has been no report of any transgenic animal that is significantly susceptible to HIV infection as a result of human CD4 and CXCR4 coexpression.
Without an effective vaccine, the number of individuals infected with HIV will likely increase substantially. Furthermore, in the absence of effective therapy, most individuals infected with HIV will develop acquired immune deficiency syndrome (AIDS) and succumb to either opportunistic infections and malignancies that result from the deterioration of the immune system, or the direct pathogenic effects of the virus. Despite the present availability of some anti-HIV agents that slow disease progression, a pressing need remains for more effective therapeutics and drug combinations. To date, there has been no report of any anti-HIV therapeutic that relates to CXCR4.
It is apparent from the foregoing that a need exists for in vitro and in vivo models suitable to the study of HIV infection and the effectiveness of anti-HIV therapeutics. By the same token, the need remains for more effective anti-HIV therapeutics. Although CXCR4 is a member of the known 7-transmembrane segment superfamily of G-protein-coupled cell surface molecules, the essential role of CXCR4 in the membrane fusion step of HIV infection was not elucidated heretofore.